Messages in this thread | | | Date | Wed, 21 Aug 2019 15:50:33 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [RT PATCH 1/3] hrtimer: Use READ_ONCE to access timer->base in hrimer_grab_expiry_lock() |
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On Wed, 21 Aug 2019, Sebastian Andrzej Siewior wrote:
> On 2019-08-21 10:24:07 [+0100], Julien Grall wrote: > > The update to timer->base is protected by the base->cpu_base->lock(). > > However, hrtimer_grab_expirty_lock() does not access it with the lock. > > > > So it would theorically be possible to have timer->base changed under > > our feet. We need to prevent the compiler to refetch timer->base so the > > check and the access is performed on the same base. > > It is not a problem if the timer's bases changes. We get here because we > want to help the timer to complete its callback. > The base can only change if the timer gets re-armed on another CPU which > means is completed callback. In every case we can cancel the timer on > the next iteration.
It _IS_ a problem when the base changes and the compiler reloads
CPU0 CPU1 base = timer->base;
lock(base->....); switch base
reload base = timer->base;
unlock(base->....);
See?
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